To synchronize your system clock just once, without starting the NTP daemon, run:

To synchronize your system clock just once, without starting the NTP daemon, run:

−

# ntpd -qg

+

# ntpd -q

{{Note|This has the same effect as the [http://support.ntp.org/bin/view/Dev/DeprecatingNtpdate now deprecated] {{ic|ntpdate}}.}}

{{Note|This has the same effect as the [http://support.ntp.org/bin/view/Dev/DeprecatingNtpdate now deprecated] {{ic|ntpdate}}.}}

−

−

The {{ic|-g}} option allows shifting the clock further than the panic threshold (15 min by default) without a warning. Note that such offset is abnormal and might indicate either wrong timezone setting, clock chip failure, or simply a very long period of neglect. If in these cases you would rather not set the clock and print an error to syslog, remove {{ic|-g}}.

After updating the system clock, store the time to the hardware clock so that it is preserved when rebooting:

After updating the system clock, store the time to the hardware clock so that it is preserved when rebooting:

Line 98:

Line 97:

===Synchronize once per boot===

===Synchronize once per boot===

−

{{Warning|Using this method is discouraged on servers and in general on machines that need to run continuously for more than 2 or 3 days, as the system clock will be updated only once at boot time.}}

+

{{Warning|Using this method is discouraged on servers and in general on machines that run without rebooting for more than a few days.}}

Write a ''oneshot'' [[systemd]] unit:

Write a ''oneshot'' [[systemd]] unit:

Line 108:

Line 107:

[Service]

[Service]

Type=oneshot

Type=oneshot

−

ExecStart=/usr/bin/ntpd -q -g -u ntp:ntp ; /sbin/hwclock -w

+

ExecStart=/usr/bin/ntpd -g -u ntp:ntp ; /usr/bin/hwclock -w

[Install]

[Install]

Line 114:

Line 113:

and enable it: {{bc|# systemctl enable ntp-once}}

and enable it: {{bc|# systemctl enable ntp-once}}

+

Note that a [[systemd]] unit of the type ''oneshot'' executes once only. Hence the {{ic|ntpd -q}} option should not be used in this case.

==Running as a daemon==

==Running as a daemon==

Line 127:

Line 127:

===Check whether the daemon is synchronizing correctly===

===Check whether the daemon is synchronizing correctly===

−

Before you can use the ntpq command you'll need to use pacman and install the libedit package. Then use ntpq to see the list of configured peers:

+

Use ntpq to see the list of configured peers:

{{bc|$ ntpq -np}}

{{bc|$ ntpq -np}}

The delay, offset and jitter columns should be non-zero. The servers ntpd is synchronizing with are prefixed by an asterisk. It can take several minutes before ntpd selects a server to synchronize with; try checking after 17 minutes (1024 seconds).

The delay, offset and jitter columns should be non-zero. The servers ntpd is synchronizing with are prefixed by an asterisk. It can take several minutes before ntpd selects a server to synchronize with; try checking after 17 minutes (1024 seconds).

+

+

===Netctl===

+

To synchronize your system clock along with a network connection through the use with [[Netctl]]. You can append the following line to your netctl profile.

+

{{bc|<nowiki>ExecUpPost='/usr/bin/ntpd -q || true'</nowiki>}}

===NetworkManager===

===NetworkManager===

Line 136:

Line 140:

===Running in a chroot===

===Running in a chroot===

+

{{Out of date|{{ic|ntpd.service}} does not use {{ic|/etc/conf.d/ntpd.conf}} anymore}}

{{Note|ntpd should be run as non-root before attempting to jail it in a chroot (default in the vanilla Arch Linux package), since chroots are relatively useless at securing processes running as root.}}

{{Note|ntpd should be run as non-root before attempting to jail it in a chroot (default in the vanilla Arch Linux package), since chroots are relatively useless at securing processes running as root.}}

Installation

Configuration

Tip: The ntp package is installed with a default /etc/ntp.conf that should make NTPd work without requiring custom configuration.

Configuring connection to NTP servers

The first thing you define in your /etc/ntp.conf is the servers your machine will synchronize to.

NTP servers are classified in a hierarchical system with many levels called strata: the devices which are considered independent time sources are classified as stratum 0 sources; the servers directly connected to stratum 0 devices are classified as stratum 1 sources; servers connected to stratum 1 sources are then classified as stratum 2 sources and so on.

It has to be understood that a server's stratum cannot be taken as an indication of its accuracy or reliability. Typically, stratum 2 servers are used for general synchronization purposes: if you do not already know the servers you are going to connect to, you should use the pool.ntp.org servers (alternate link) and choose the server pool that is closest to your location.

The iburst option is recommended, and sends a burst of packets only if it cannot obtain a connection with the first attempt. The burst option always does this, even on the first attempt, and should never be used without explicit permission and may result in blacklisting.

Configuring your own NTP server

If setting up an NTP server, you need to add local clock as a server, so that, in case it loses internet access, it will continue serving time to the network; add local clock as a stratum 10 server (using the fudge command) so that it will never be used unless internet access is lost:

server 127.127.1.0
fudge 127.127.1.0 stratum 10

Next, define the rules that will allow clients to connect to your service (localhost is considered a client too) using the restrict command; you should already have a line like this in your file:

restrict default nomodify nopeer noquery

This restricts everyone from modifying anything and prevents everyone from querying the status of your time server: nomodify prevents reconfiguring your ntpd (with ntpq or ntpdc), and noquery prevents dumping status data from your ntpd (also with ntpq or ntpdc).

You can also add other options:

restrict default kod nomodify notrap nopeer noquery

Note: This still allows other people to query your time server. You need to add noserve to stop serving time. It will also block time synchronization since it blocks all packets except ntpq and ntpdc queries.

Note that a systemd unit of the type oneshot executes once only. Hence the ntpd -q option should not be used in this case.

Running as a daemon

To start ntpd:

# systemctl start ntpd

To enable ntpd at startup:

# systemctl enable ntpd

Or alternatively with the command:

# timedatectl set-ntp 1

Check whether the daemon is synchronizing correctly

Use ntpq to see the list of configured peers:

$ ntpq -np

The delay, offset and jitter columns should be non-zero. The servers ntpd is synchronizing with are prefixed by an asterisk. It can take several minutes before ntpd selects a server to synchronize with; try checking after 17 minutes (1024 seconds).

Netctl

To synchronize your system clock along with a network connection through the use with Netctl. You can append the following line to your netctl profile.

ExecUpPost='/usr/bin/ntpd -q || true'

NetworkManager

Note: ntpd should still be running when the network is down if the hwclock daemon is disabled, so you should not use this.

It is relatively difficult to be sure that your driftfile configuration is actually working without waiting a while, as ntpd does not read or write it very often. If you get it wrong, it will log an error; if you get it right, it will update the timestamp. If you do not see any errors about it after a full day of running, and the timestamp is updated, you should be confident of success.

Alternatives

An alternative to NTPd is Chrony, a dial-up friendly and specifically designed for systems that are not online all the time.